Navigation systems such as instrument landing systems (ILS) are often used for guiding vehicles, particularly aircraft in the vicinity of an aircraft runway. It has been found that the use of scanning beams of radiation have an advantage over fixedly oriented beams of radiation for communicating with aircraft at low elevations since the scanning procedure tends to average out perturbations in the directivity pattern of such radiation caused by fluctuations in the terrain in the vicinity of an antenna providing a scanning beam. Furthermore, a scanned beam can communicate with aircraft having differing azimuthal directions around an antenna radiating this beam, thereby greatly increasing the capacity of an instrument landing system in simultaneously guiding many aircraft. An aircraft illuminated by a beam of radiation in the typical instrument landing system needs position data of the beam, and in the situation where the beam is being scanned, such position data must be continuously and accurately updated at a rate commensurate with the rate of motion of the beam. A problem arises in that modern aircraft fly at high speeds thereby requiring a high rate of position data with great accuracy. Obtaining the necessary accuracy has heretofore been hindered by bandwidth limitations and the desirability of maintaining relative simplicity of equipment on board the vehicle which is being navigated.